The number of power consumers in modern vehicles as well as the power consumption thereof continues to increase. That is why it is being considered to equip vehicles with a 48 V on-board power supply which is able to supply different consumers (loads) in the vehicle with a greater amount of power at steady current strengths. The use of 48 V leads to the problem, however, that where there is damage by a short circuit, stable arcs can form since 48 V is above the arc ignition voltage. However, the formation of stable arcs not only occurs in 48 V on-board power supplies in motor vehicles but rather in on-board power supplies in general which are operated at a voltage above the arc ignition voltage, such as, for example, in airplanes, ships or rail vehicles.
Traditionally, electrical wires in an on-board power supply are protected by safety fuses. However, since an arc acts as an additional resistor in the wire, the short-circuit current is restricted such that the fuse is not triggered.
In general, a distinction is made between two types of arcs, namely serial and parallel arcs, which will be explained in more detail below.
FIG. 1 shows an example of a parallel arc and in particular a switching arrangement in an on-board power supply with a 48 V voltage source and a current profile over time. Parallel arcs occur parallel to the load. These are caused, for example, by defective cable insulation such that a short circuit occurs to the body or the existing 12 V on-board power supply. With parallel arcs, the current strength has a succession of spikes that have several hundred amperes. Since the spikes can be short, however, the mean current is often not sufficient to trigger the fuse.
FIG. 2 shows an example of a serial arc and in particular a switching arrangement in an on-board power supply with a 48 V voltage source. Serial arcs occur in series to the load. These are caused, for example, by cable breakage or damage to plug-in connectors. Serial arcs act as an additional resistor in the electrical circuit such that the current strength is reduced in comparison to the intact state of the electrical circuit. Consequently, a serial arc restricts the load current such that a fuse is not triggered.
Both serial and parallel arcs generate very high temperatures such that a stable arc can cause serious damage to the vehicle. Consequently, it is desirable to recognize early on the emergence of an arc. It is very difficult, however, to distinguish the current restriction by a serial arc from fluctuations in the decrease in current by the load. In particular, protection of wires in a 48 V on-board power supply cannot be carried out via safety fuses since the current through the fuse is reduced by the arc effect such that the fuse is not triggered.
Recognizing disruptions (e.g. high-resistance short circuits) creates specific problems which occur, for example, during the slow chafing of the wire and/or electromigration. The short-circuit arc currents are hard to recognize here since they can be located in the load region. On the other hand, the arc power is not that high, and therefore recognition may require more time.
Known possibilities for detecting parallel and serial arcs are described below.
With parallel arcs, part of the current flows across an arc to the mass via a short circuit parallel to the load (see FIG. 1). The current Iin fed into the wire is thus Iload+Iarc. The parallel short circuit with arcs can therefore be recognized by a differential current measurement. The current in the wire Iin and the current to the load Iload are measured. Without a parallel short circuit, Iload=Iin. In the case of the parallel short circuit, Iarc=Iin−Iload can be calculated. Accordingly, the condition for recognition is Iin−Iload>0.
DE 10 2012 023 460 A1 describes such a method for recognizing a parallel arc in a motor vehicle on-board power supply. The motor vehicle on-board power supply comprises a component path in the network area, the component path having at least one electrical component. The motor vehicle on-board power supply has a first and a second current measuring unit in the component path to measure the current in each component path. The first current measuring unit measures the current at a first measuring point and the second current measuring unit at a second measuring point in the component path. To monitor the component path as to the emergence of arcs, the motor vehicle on-board power supply comprises at least one arc monitoring unit for recognizing an arc by a difference in current between the current measured in the first current measuring unit and the current measured in the second current measuring unit.
With serial arcs, an arc is formed in series to the load by an interruption in the electrical circuit (e.g. line breakage). Owing to the voltage drop across the arc, Uin−Uload=I×Zwire+Uarc applies. Since the arc voltage with Uarc, typically above 15 V, is very high, the drop in voltage across the wire resistance Zwire is negligible in the first approximation and U_in−Uload>15 V applies as the condition for the recognition of a serial arc.
The parallel arc can thus be recognized by a differential current measurement at the beginning and the end of the wire and the serial arc can be recognized by a differential current measurement.
If the current and voltage measurement at the beginning of the wire occurs in a current distributor and the measurement of current and voltage at the end of the wire occurs in a load, communication of the measured signals to an evaluation unit must occur.
However, it is problematic with this approach that the measurements in the load and in the feeding current distributor occur independent of one another and the measurement values must first be communicated via a bus system. The measurement values are therefore asynchronous and delayed in relation to one another. Synchronization of the measurements and taking into account the time delay are technically very complex. Even without arcs, the current distributor can detect a peak value during its cyclical sampling, whereas at the load it can detect, somewhat delayed, precisely a local minimum. The simple differential formation would thus generate a pseudo error. This applies in particular when high-resistance short circuits with currents in the load range are supposed to be recognized.
FIG. 3 shows an example of a system in which measurement values are asynchronous and delayed in relation to one another. The system comprises a load which is supplied with power from a battery via a supply line. The current from a current distributor is distributed thereby to different loads. The load is an intelligent load, i.e. the load comprises an electronic circuit which measures the voltage and current strength on the load. The measured values are sent by means of a serial interface to the current distributor via a communication channel.
The current distributor comprises a reception unit which receives the measurement values sent by the load and relays them to an evaluation unit. The current distributor is further configured to measure the current flowing through the current distributor and the voltage applied to the current distributor. These measurement values are also detected by the evaluation unit. Based on the measurement values received, the evaluation unit recognizes the occurrence of an arc.
FIG. 4 shows the problem of the asynchronous and delayed arrival via the communication channel of the data in the evaluation unit. The upper part of FIG. 4 shows the temporal profile of a measured quantity, such as, for example, the current strength at the current distributor and the load without an arc.
The central part of FIG. 4 shows the measuring point in time (tsv) in the current distributor. The current distributor measures every 10 ms.
The lower part of FIG. 4 shows the measuring point in time (tL) in the load. The load measures every 8 ms, thus the sampling intervals differentiate between the current distributor and the load. Additionally, the current distributor and the load do not start their measurements simultaneously but instead offset by 2 ms. Thus, the measurements do not occur synchronously.
Furthermore, there are time delays owing to the communication channel such that the measurement values are not simultaneously present in the current distributor. It is assumed in FIG. 4 that the communication channel leads to a delay of 20 ms. The current distributor performs a determination of an arc every 5 ms based on the difference of the present measurement values. In the current distributor the measurement value tsv0 is found in the 0-4 ms interval and the measurement value tsv10 is found in the 10-14 ms interval. The values tsv20 and tL2 are found in the 20-24 ms interval, the values tsv30 and tL10 are found in the 30-34 ms interval, the value tL18 is found in the 35-19 ms interval, the values tsv40 and tL26 are found in the 40-44 ms interval and the values tsv50 and tL34 are found in the 50-55 ms interval. If the measurement values within an interval have a difference larger than a threshold value, the evaluation unit determines the presence of an arc. However, this determination can be erroneous owing to the asynchronicity and the time delay if the measured quantity varies greatly.